1. Field of the Invention
The present invention relates to a CDMA radio transmitting apparatus and a CDMA radio receiving apparatus used for digital radio communications, etc.
2. Description of the Related Art
One of the line connection systems in digital radio communications is the multiple access system in which a plurality of stations can perform communications with the same frequency band simultaneously. A type of the multiple access system that allows improvement of frequency utilization is the CDMA system.
CDMA (Code Division Multiple Access) means code division multiple accesses and refers to a technology that achieves multiple accesses through spread spectrum combinations in which an information signal spectrum is spread in a band sufficiently wide compared to the original information band width. It is also sometimes called spread spectrum multiple access (SSMA). The system in which a spread code is directly multiplied by an information signal is called "direct sequence system."
FIG.1 shows an outline of configuration example of a conventional CDMA transmitting apparatus. With the CDMA transmitting apparatus shown in the figure, variable data 1201 is assembled by frame assembly circuit 1202 in frame units, and then time-multiplexed with fixed data 1204 by slot assembly circuit 1203. At this time, slot timing, symbol timing and data rate information 1205, etc. are input to slot assembly circuit 1203 to control the time-multiplexing timing. FIG.2 shows the configuration of the slot assembly circuit. Transmit data 1301 which is variable data and fixed data 1204 are time-multiplexed and output by switch 1303 which is controlled by timing control circuit 1302. In slot assembly circuit 1203, a slot-assembled signal is primary-modulated by modulator 1206, CDMA-modulated by spread circuit 1207, and then amplified by RF section 1208 and transmitted from antenna 1209.
The data format of radio signals used for the CDMA transmitting apparatus above is explained below.
FIG.3 shows an example of radio signal format. A transmit signal includes slots as its basic units, with K slots making up one frame and N frames making up one super frame.
FIG.4 shows an example of transmit signal format in one slot. One slot includes a fixed data section with the quantity of data constant with time and a variable data section with the quantity of data variable with time.
The fixed data includes a pilot symbol which is a known signal for coherent detection on the receiving side, control signal such as power control signal, or rate information of the variable data section transmitting the quantity of data, etc. The variable data is coding data with the quantity of data variable with time such as voice information and image information.
The slot shown in FIG. 4(a) indicates a slot configuration when the data rate is high such as voice period; FIG. 4(b), when there is no data such as silent period; and FIGS. 4(c) and (d), when there is little data such as when the data rate is low.
FIG. 5 shows the power of the conventional CDMA transmitting apparatus for each frame. For example, if the data rate of transmit data is high, it transmits variable data 1602 with the same power as that of fixed data 1601 as shown in FIG. 4(a). If there is no data as shown in FIG. 5(b), fixed data 1603 is transmitted in the same way as in FIG. 5(a), while variable data 1604 is set to power 0. Furthermore, when the data rate is low as shown in FIG. 5(c), fixed data 1605 is transmitted in the same way as in FIG. 5(a), whereas variable data 1606 is transmitted with small power instead of transmitting the same signal repeatedly. This allows the quality of variable data 1606 to be kept equivalent to the quality of fixed data 1605. At the same time, transmitting variable data 1606 with low power can reduce interference with other users in that portion of data. Furthermore, when the data rate is low as shown in FIG. 5(d), fixed data 1607 is transmitted in the same way as in FIG. 5(a), and variable data 1608 is also transmitted with the same power, but can also be cut midway if there is little data. This allows the quality of variable data 1608 to be kept equivalent to the quality of fixed data 1607.
When the data rate is high, this results in a power pattern as shown in FIG. 5(a); when there is no data, a power pattern as shown in FIG. 5(b); and when the data rate is low, power patterns as shown in FIGS. 5(c) or (d).
On the other hand, in the CDMA receiving apparatus on the receiving side, as shown in FIG. 6, the signal received by an antenna 1701 is down-converted by RF circuit 1702 and then despread by despread circuit 1703, demodulated by demodulator 1704 and separated by slot disassembly circuit 1705 into the fixed data section and variable data section. Frame disassembly circuit 1707 output the variable data as receive data. In slot disassembly circuit 1705 as shown in FIG. 7, demodulator output 1801 resulting from time-multiplexing of the variable data and fixed data is separated into fixed data 1805 and variable data 1806 by switch 1804 controlled by timing control circuit 1802 using slot timing, symbol timing and data rate information, etc. 1803.
However, as shown in FIGS. 5 (b) and (c), with the conventional transmitting apparatus above, when there is no data or the data rate is low, turning transmit energy ON/OFF will generate a pulse signal with a power pattern of specific cycles, producing line spectrums with large power in specific frequency components, which will get mixed in hearing aids causing unnecessary sound with a specific frequency, a so-called hearing aid problem, or may affect peripheral appliances.
FIG. 8 shows an example of frequency spectrum when transmit power is turned ON/OFF in a cycle of 1.6 kHz. In this case, line spectrums generating unnecessary sound are observed in the audible range such as 1.6 kHz and 3.2 kHz.